1. Field of the Invention
The present invention relates generally to a shock sensor and method for monitoring shock. More particularly, the present invention relates to a low-power and unpowered micro-electromechanical shock sensor using a micromechanical suspended proof mass structure.
2. Background of the Invention
Embedding miniature sensors in products, systems, storage and shipping containers, and other items allows the monitoring of those items to determine health, maintenance needs, lifetime, and other item characteristics. Information from miniature shock sensors can tell a user whether the item has been exposed to shock levels that can cause damage. In addition, miniature shock sensors can be used to “wake up,” from a low-power sleep mode, a more sophisticated sensing system to collect a more complete set of environmental data.
Current battery-powered embedded sensor systems that perform this type of monitoring often require a low power method of determining when a certain level of shock has been reached. Many other applications, such as in transportation and shipping monitoring, heating and air conditioning, and food storage, would benefit from the ability to monitor the shock environment with a completely unpowered sensor. In addition, these applications would benefit from the ability to poll that sensor to determine if a shock extreme was reached, and then reset the sensor for later use. In either case, an ultra-low power sensor, or even a sensor that consumes no quiescent power, would reduce the overall system power consumption enough to allow embedded sensors to operate for many years in portable battery powered applications, or in systems that scavenge small amounts of power from the environment.
Low power and unpowered shock sensors currently exist. However, they are large-scale devices such as the catches used in automotive seat belts. These devices operate in a similar fashion and provide a similar function as the present invention, but are not in a form factor suitable for integration with microdevices, and are not fabricated using techniques that are compatible with microelectronics or micro-electromechanical systems (“MEMS”) devices.
Micro-scale shock sensors, in the form of accelerometers, exist as well, but most of the previous work to develop low-power shock sensors has been focused on minimizing the power consumption of standard miniature devices, and using low-power analog electronics to determine when a specific shock level has been reached. Devices and systems would then create a low-impedance logic level signal for input to a sleeping microcontroller. The fundamental problem is that such a system must continuously power the sensor and analog trigger circuitry, creating a constant power draw on the batteries. Even using the latest in low-power devices and highest capacity batteries, systems that continuously power any sensor will only operate for 5-10 years.
As embedded miniature sensors get smaller, and as batteries are reduced in size and capacity, the use of lower power and unpowered devices will become more critical. Furthermore, maximizing the sensor functionality, without increasing power consumption, will enhance the capability of embedded sensing systems.
Other inventions have used suspended proof mass micro-machined devices to measure shock, and for switching, but, until the present invention, only one as had the advantages of the present invention in combining low- or no-power operation with a mechanical latching function. U.S. Pat. No. 6,737,979 discloses a MEMS shock sensor that achieves the goals of low- and no-power operation of a mechanical shock sensor with a mechanical latching function. In this prior art invention, as in the present invention, a moveable proof mass and a latching means are formed on the surface of a substrate. When the sensor is subject to a sufficient shock, the proof mass moves and latches with the latching means, and the latched condition is detected by external circuitry.
The present invention offers several improvements to the technology disclosed in U.S. Pat. No. 6,737,979 (“the '979 invention”). First, in the '979 invention, each separate device design can detect only one range of shock level because the distance between the proof mass and the latch is not variable. In the present invention, the latching distance is variable and a sensor can therefore be programmed to detect varying shock levels. Second, in the '979 invention, the only electrical contact made between the proof mass and the latch to detect a shock level is through the latch itself. As is discussed in detail below, the present invention offers a contact that is separate from the latch so that a “triggering” condition (i.e., the proof mass contacting with the contact) can be made (and detected) prior to latching, if desired by the user. With this feature, the present invention can be programmed to detect a shock level smaller than that of the latching shock level. Third, although the '979 invention offers an unlatching function so that the sensor can be re-used, the present invention improves upon this function with a mechanical linkage that applies no load to the latch during latching, thereby decreasing the necessary latching force and increasing the sensitivity of the sensor.